1. Field of the Invention
The present invention relates to sintered substrates used for alkaline storage batteries and to alkaline storage batteries using the substrates.
2. Description of Related Art
Alkaline storage batteries have been used as power sources for apparatus that require high power, such as power tools and hybrid electric automobiles in recent years.
Typical examples of electrode plates for the alkaline storage batteries are sintered electrode plates, which are obtained by filling an active material into pores in a porous sintered substrate in which nickel powder is sintered on both sides of a perforated steel plate or the like, and non-sintered electrode plates, which are obtained by applying a paste prepared by mixing an active material powder with a binder agent onto a conductive substrate such as a perforated steel plate and a nickel foam. The sintered electrode plates are excellent in conductivity because of the sintered nickel and are therefore more advantageous for batteries that require high power as mentioned above.
The sintered electrode plates are generally manufactured in the following manner.
First, a slurry is prepared by kneading nickel powder, a thickening agent such as methyl cellulose, and water in predetermined proportions. Then, the resultant slurry is coated onto the surface of a conductive core such as a perforated steel plate, and a sintering process is performed by heating the material in a reducing atmosphere. A sintered substrate is thus obtained.
By filling an active material into the resultant sintered substrate, a sintered electrode plate is obtained. In the case of a sintered cadmium negative electrode, for example, the sintered substrate is immersed in a solution containing cadmium nitrate as its main component so that cadmium nitrate can be held in the pores, and then the sintered substrate is immersed in an alkaline solution, such as sodium hydroxide, to thereby convert the cadmium nitrate in the pores into cadmium hydroxide. This process is repeated a necessary number of times to fill a predetermined amount of cadmium hydroxide into the pores of the sintered substrate. Thus, the sintered cadmium negative electrode is obtained.
When the pores occupy a greater proportion (i.e., the porosity is greater) in a sintered substrate, a greater amount of active material can be filled in the sintered substrate correspondingly and the energy density of the electrode plate can be increased. For this reason, a pore forming agent made of microcapsules using an organic shell is added to the above-described slurry in order to increase the porosity of the sintered substrate.
On the other hand, increasing the porosity can lead to a poorer strength of the sintered substance. When the strength lowers, peeling may occur in the subsequent electrode coiling process step or the like, which can become a cause of short circuiting after the battery is assembled. Thus, there is a demand for a sintered substrate that has an improved porosity and also maintains a certain degree of strength.
A reason for the lower strength that results from increasing the porosity by the addition of a pore forming agent is believed to be that formation of large-sized pores originating form the pore forming agent occurs. Such large-sized pores do not proportionately increase the amount of the active material that can be filled in the substrate relative to the increase of the porosity since the active material is not easily held in such large-sized pores. Moreover, such large-sized pores may result in a poorer conductivity of the sintered substrate and consequently deteriorates the utilization factor.
In view of these problems, Japanese Published Unexamined Patent Application Nos. H4-248269 and 2003-297371, for example, propose methods of reducing the large-sized pores in the sintered substrate to make the pore size distribution uniform. By reducing the number of the large-sized pores, an improvement in the porosity and maintaining of the strength can be accomplished at the same time.
However, as further higher performance electrodes have been demanded, it has become necessary to develop a sintered substrate in which the number of the large-sized pores is less and the pore size distribution is more uniform.
The present invention has been accomplished in view of the foregoing circumstances, and it is an object of the invention to provide a sintered substrate having a more uniform pore size distribution than the conventional ones, and to provide an alkaline storage battery using the substrate.